The battle of Katasyrtai occurred in the fall of 917, shortly after the striking Bulgarian triumph at Acheloos near the village of the same names close to the Byzantine capital Constantinople, (now Istanbul). The result was a Bulgarian victory.
Origins of the conflict 
From the beginning of 917 both sides prepared for decisive actions. The Byzantines tried to forge a coalition against Bulgaria but their attempts failed due to the fast reaction of Simeon. Nonetheless the Byzantines gathered an enormous army but they were decisively defeated at Acheloos.
The battle 
While the victorious Bulgarian army was marching southwards, the Byzantine commander Leo Phokas, who survived at Acheloos, reached Constantinople by sea and gathered the last Byzantine troops to intercept his enemy before reaching the capital. The two armies met near the village of Katasyrtai just outside the city and after a night fighting, the Byzantines were completely routed from the battlefield.
The last Byzantine military forces were literally destroyed and the way to Constantinople was opened but the Serbs rebelled to the west and the Bulgarians decided to secure their rear before the final assault of the Byzantine capital which gave the enemy precious time to recover.
- Ioannes Scylitzes. Historia. 2, p.88